Historically, computing systems were rather large and cumbersome devices that required power from an alternating current source, such as a 110 volt wall outlet, to operate. More recently, with the advent of miniaturized circuits, memories and processing devices, computing systems have dramatically increased their computing power density, meaning that more and more powerful computing devices are available in smaller and smaller form factors. While ten years ago a conventional computing device was constructed in a desktop or mainframe form factor, today much more powerful computing devices are available in notebook and even hand-held form factors.
As the form factors of computing devices have shrunk considerably and the portability of such devices has become desirable, it has become imperative that such devices be unteathered from conventional wall outlets. Accordingly, notebook computing devices routinely include batteries, which are charged by wall outlet power, that enable use of the notebook computing device for several hours without the need for wall outlet power. Similarly, hand-held devices such as personal digital assistants (PDAs) operate almost exclusively on batteries.
Battery life of computing devices has, therefore, become a major consideration. One significant power consumer in a computing device is the processor utilized by the device. To make the processor operate as efficiently as possible, many processors of computing devices include an Advanced Computer Power Interface (ACPI), which is a basic input/output system (BIOS)-based power management system. ACPI uses device activity timeouts to determine when to transition a processor into one or more low power, or sleep, states in which the processor consumes a reduced quantity of current.
For example, the ACPI specification defines five sleep states that are referred to as S1-S5. In nearly all of the sleep states, the processor, cache and chip set contexts are lost, but are restored by the processor upon exit of the sleep state by recalling information stored prior to entering the sleep state. In typical operation, after a pre-defined period of computing system inactivity the ACPI, which operates unbeknownst to the OS, cause the processor to go to sleep. Prior to entering sleep state, the OS writes to an ACPI non-volatile storage (NVS) memory, information such as code and data that the processor will need to restore state information upon awakening from the sleep state. After the necessary code and data are written to the ACPI NVS, the processor goes into the sleep state, during which processor current consumption is reduced and the operation of the OS is suspended. While in the sleep state, the basic input/output system (BIOS) monitors the system for activity. When the BIOS detects activity, such as keyboard activity or some other input activity, the BIOS causes the processor to execute a resume process that causes the processor to exit the sleep mode and to use the information stored in the ACPI NVS to restore the processor to an active operation state.
Conventionally, there is a trusting relationship between the BIOS and the OS with respect to a resume operation. In particular, the resume information stored in the ACPI NVS is treated by the BIOS as being reserved and it is assumed that a post-boot agent did not attack or corrupt the ACPI NVS contents.